The present invention relates to cordierite ceramics, and more particularly to cordierite ceramics produced by the firing of chemically modified mineral batches for such ceramics.
The production of cordierite (2MgO-2Al.sub.2 O.sub.3 -5SiO.sub.2) ceramics from mineral batches containing mineral sources of magnesium alumina, and silica such as clay and talc is well known. Such processes are described, for example, in U.S. Pat. No. 2,864,919. The manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudate to provide ceramics with very low thermal expansion coefficients along at least one axis, is disclosed in U.S. Pat. No. 3,885,977.
The selection of mineral batch materials to be used in the production of cordierite ceramics by extrusion has been found to significantly affect the final properties of the resulting ceramics. For example, U.S. Pat. No. 4,280,845 describes the effects of varying the particle size and porosity of talc on the thermal shock resistance and porosity of the resulting cordierite ceramics.
It has been proposed to utilize extruded honeycomb materials composed of cordierite in rotary heat exchangers (regenerators) for gas turbine and other heat engines. For best performance as a regenerator, high strength in combination with low porosity and high thermal shock resistance are required. U.S. Pat. Nos. 4,877,670 and 5,030,398 describe cordierite ceramics exhibiting improved characteristics for these applications, produced by utilizing mineral (talc and clay) batch components of small and carefully defined particle sizes. The products provided in accordance with these patents reportedly exhibit thermal expansion coefficients (CTE values) in the 5-12.times.10.sup.-7 /.degree.C. range, typically with total porosities of 25-35 volume percent.
While such ceramics represent an improvement in properties over extruded cordierite ceramics produced using more conventional mineral sources, still further improvements in these products, particularly with respect to thermal expansion, porosity and strength, would be desirable. This is because heat regenerators and other components exposed to continuous heating and cooling in normal use must exhibit exceptionally high thermal durability in order to insure that reasonable service life will be attained.
One of the problems with existing extruded cordierite ceramics is that they exhibit only limited thermal durability under repeated thermal cycling. Hence, when thermally cycled within and above the 600.degree.-800.degree. C. temperature range during exposure to hot flowing exhaust gases, these materials can demonstrate significant strength reductions, particularly in regions near the hot face of the regenerator subjected to the highest temperatures.
It is therefore a principal object of the present invention to provide improved cordierite ceramics, and a method for making them, such that the ceramics exhibit higher strength in combination with lower overall porosity, for the purpose of achieving improved durability in a high temperature thermal cycling environment.
It is a further object of the invention to provide a method which utilizes modified ceramic batch compositions to produce extruded cordierite ceramics with improved properties.
Other objects and advantages of the invention will become apparent from the following description thereof.